Solid electrolyte ionic conductors, also referred to as ion transport membranes, can be utilized to separate oxygen from gas mixtures containing oxygen. Mixed conductors are materials that conduct both oxygen ions and electrons and appear to be well suited for oxygen separation since they can be operated in a pressure driven mode, in which oxygen transport is driven by a difference in oxygen activity, also referred to as oxygen partial pressure, on the two sides of the membrane. Perovskites such as La.sub.1-x Sr.sub.x CoO.sub.3-y, La.sub.x Sr.sub.1-x FeO.sub.3-y, and La.sub.x Sr.sub.1-x Fe.sub.1-y Co.sub.y O.sub.3-z are examples of mixed conductors. At elevated temperatures, these materials contain mobile oxygen-ion vacancies that provide conduction sites for transport of oxygen ions through the material. These materials transport oxygen ions selectively, and can thus act as a membrane with an infinite selectivity for oxygen.
Thin electrolyte films are highly desirable because the ideal oxygen flux is inversely proportional to the thickness. Thus thinner films could lead to higher oxygen fluxes, reduced area, lower operating temperatures and smaller oxygen pressure differentials across the electrolyte.
Solid state gas separation membranes formed by depositing a dense mixed conducting oxide layer onto a relatively thick porous mixed conducting support were investigated by Teraoka et. al. as disclosed in the Journal Ceram. Soc. Japan, International Ed, Vol. 97, No. 5 (1989). The relatively thick porous mixed conducting support provides mechanical stability for the thin, relatively fragile, dense mixed conducting layers. La.sub.0.6 Sr.sub.0.4 CoO.sub.3 thin films were deposited onto porous supports of the same material by an rf sputtering technique and a liquid suspension spray deposition method. Films produced by the sputtering method were cracked and porous. Films (approximately 15 .mu.m thick) made by the liquid suspension spray followed by sintering at 1400.degree. C. were dense and crack-free.
Teraoka and coworkers expected the oxygen flux to increase by a factor of 10 for the composite thin film membrane compared to a dense disk. However, they obtained an increase of less than a factor of two.
Pal et al. disclosed an EVD process in a paper entitled "Electrochemical Vapor Deposition of Yttria-Stabilized Zirconia Films" wherein a yttria-stabilized zirconia ("YSZ") film is deposited onto a porous substrate. EVD is a modification of the conventional chemical vapor deposition ("CVD") process which utilizes a chemical potential gradient to grow thin, gas impervious metal oxide films on porous substrates. The EVD process involves contacting a mixture of metal halides on one side of a porous substrate and a mixture of hydrogen and water on the opposite side. The reactants diffuse into the substrate pores and react to form the multi-component metal oxide which is deposited on the pore wall. Continued deposition causes pore narrowing until eventually the pores become plugged with the multi-component metal oxide. The primary application of EVD to date has been in the fabrication of solid electrolyte YSZ, and the interconnector material lanthanum chromium oxides as used in solid oxide fuel cells ("SOFCs").
Richards et al. in U.S. Pat. No. 5,240,480 disclosed an organometallic chemical deposition (OMCVD) method to prepare thin films of muti-component metallic oxides for use as inorganic membranes. The inorganic membranes are formed by reacting organometallic complexes corresponding to each of the respective metals and an oxidizing agent under conditions sufficient to deposit a thin membrane onto the porous substrate. Both EVD and OMCVD process involve expensive and complex equipment and often toxic and expensive precursor materials. Furthermore, for multi-component metallic oxides (e.g. mixed conducting perovskites), stoichiometry control of the oxide film is difficult for these processes.
Thorogood et. al. in U.S. Pat. No. 5,240,480 investigated multi-layer composite solid state membranes which are capable of separating oxygen from oxygen-containing gaseous mixtures at elevated temperatures. The membranes comprise a multi-component metallic oxide porous layer having an average pore radius of less than approximately 10 .mu.m and a multi-component metallic oxide dense layer having no connected-through porosity wherein the porous and dense layers are contiguous and such layers conduct electrons and oxygen ions at operating temperatures.
Carolan et al. in U.S. Pat. No. 5,569,633 investigated surface catalyzed multi-layer ion transport membranes consisting of a dense mixed conducting multi-component metallic oxide layer, and combinations of porous ion conducting and porous mixed ion and electron conducting layers. Significant oxygen flux was demonstrated by these prior art ion transport membranes in which catalysts were deposited onto the oxidizing surface of the composite membrane. Coating on both sides of the membrane did not enhance the oxygen flux.
Anderson et al. in U.S. Pat. No. 5,494,700, which is incorporated herein by reference, disclose synthesis of a precipitate-free aqueous solution containing a metal ion and a polymerizable organic solvent to fabricate dense crack-free thin films (&lt;0.5 .mu.m/coating) on dense/porous substrates for solid oxide fuel cell and gas separation applications. First, a precipitate-free starting solution is prepared containing cations of oxide constituents dissolved in an aqueous mixture comprising a polymerizable organic solvent. The precursor film is deposited on the substrate by spin-coating technique followed by drying and calcining in the presence of oxygen and at the temperature not in excess of 600.degree. C. to convert the film of polymeric precursor into the metal oxide film.
The polymeric precursor method disclosed by Anderson et al. is a cost-effective approach and is easy to scale up for manufacturing. However, the upper thickness limit for a single coating is typically below 0.5 .mu.m for this method. Films greater than 0.5 .mu.m usually generate cracks during the organics burn-off and sintering due to the large shrinkage mismatch between the film and the substrate. Also, the Anderson et al. method is mostly confined to producing dense films on planar substrates by spin coating technique using a precipitate-free aqueous solution. No test results were reported for gas separation applications.
An additional concern during production of composite membranes, having a dense thin film membrane deposited on a porous structured substrate, is that such membranes are prone to defects including tiny "pinholes" which are produced during the manufacturing operation. In general, the defect density tends to increase with higher processing speeds. Such defects are highly undesirable because they are non-selective, that is, they indiscriminately pass undesired components of a feed fluid. Such defects lower the selectivity of the ion transport film and result in diminished performance. The elimination of defects is hence essential to develop high performance composite films which can be economically produced at high processing speed.
Furthermore, in practice the kinetics of surface exchange processes impose resistances additional to ion transport bulk resistance and affect oxygen transport across the ion transport membrane. As the film becomes thinner, the proportion of the overall resistance due to the ion transport bulk resistance decreases while that due to surface exchange increases. As a consequence, surface exchange kinetics are likely to become the dominant resistance for very thin films (e.g. 5 .mu.m or less). Therefore to get the full benefit of the thinner films, it is necessary to de-bottleneck the rate limitation imposed by the surface exchange processes. In summary, researchers are continuing their search for a cost-effective thin film technology for composite ion transport membranes which possess superior oxygen flux to enable their use in commercial processes.